MINOR: http-ana: Use a TXN flag to prevent after-response ruleset evaluation

MINOR: http-ana: Use a TXN flag to prevent after-response ruleset evaluation

The txn flag TX_CONST_REPLY may now be used to prevent after-response ruleset
evaluation. It is used if this ruleset evaluation failed on an internal error
response. Before, it was done incrementing the parameter <final>. But it is not
really convenient if an intermediary function is used to produce the
response. Using a txn flag could also be a good way to prevent after-response
ruleset evaluation in a different context.